After some thought about doors and the roles they can play, the concept of a door-knock detection device intrigued engineer Paul Mandel so much that he created a "knock box". It's a fascinating project that uses a piezo sensor to detect a knock, then repeats the sequence back using a solenoid that taps the walls of the box that contains it. For example:

That's pretty cool, and simple to make. It would also be the heart of a fun childrens' toy. Paul has published the required details so head over to his blog for more. And for more, we're on twitter and Google+, so follow us for news and product updates as well.

It can be used as a noise-maker driven by your microcontroller for audible feedback of events, and it can also be used as a knock-detector input to sense events and react to them. Includes a built-in 1M resistor to allow the piezo element to detect shocks. For more information and to order, please visit the product page here.